1. Field of the Invention
The present invention relates to a method for determining a variable, especially physiological, control parameter for an implantable medical device, particularly for the heart rate adaptation in a cardiac pacemaker.
2. Background Art
Although the present invention has applications in connection with a variety of implantable medical devices, for purposes of better illustration the background of the invention can be explained based on the example of a cardiac pacemaker.
A modern cardiac pacemaker represents an implantable medical device for which a certain physiological control parameter must be determined and set. This control parameter may be the heart rate, for instance, i.e., the frequency at which the heart beats, if necessary upon delivery of a stimulation pulse by the pacemaker. The heart rate of a pacemaker-supported heart is, accordingly, comparable to the pulse frequency of the healthy heart.
It is well known that the pulse frequency, and accordingly also the optimum heart rate as it is to be generated by a pacemaker, depends on the given current condition of the patient. If the patient is exercising, for example, the heart rate must be adjusted higher than when the patient is resting. At the same time, an increased ejection performance of the heart is attained as well, in addition to an increased heart rate or pulse frequency, due to an increased contractility. A wide-spread heart disorder, in this context, results from the fact that, on one hand the natural rate adaptation of the heart through the sinus node no longer functions, but a natural adaptation of the contraction behavior of the heart to the given current physiological needs of a patient is still present. A heart that adapts in this way changes its contraction behavior, for example by changing its beat volume or its contraction speed. As known from the prior art, this change in the contraction behavior can be measured as the starting parameter, knowing that the contraction behavior is significant for the heart rate. The intracardial impedance, for example, must now be measured with a unipolar measurement as the base parameter for this contraction behavior. This is done by applying, between a so-called tip electrode, in the top of the ventricle, and the casing of the pacemaker, a measuring current, which may consist, for example, of bi-phase rectangular pulses of a width of a few ten microseconds and a repetition frequency of some ten to a few hundred hertz. The change in the intracardial impedance due to a contraction of the heart is based on the changing ratio of high-ohm myocardial tissue to low-ohm blood in the vicinity of the tip electrode. A representative measuring signal curve for the intracardial impedance as the base parameter is obtained from a certain contraction behavior. The intracardial impedance, therefore, depicts the dynamics of the contraction process.
These measured dynamics of the contraction process are used for the rate adaptation in known pacemakers in such a way that an algorithm, which is based on clinical experience and corresponding ratings, is used to calculate the rate. This algorithm refers to the difference between the measuring signal curve of the intracardial impedance and an at-rest impedance curve as the reference signal curve. This at-rest curve is subject to fairly long-term changes, which may occur, for example, as an effect of drugs, or due to changes in the patient""s fitness level. In the art, the at-rest curve is therefore continually updated.
As was now determined as the starting point for the development of the present invention, the contraction dynamics vary during different events and successions of events, such as, on one hand, during a natural self-stimulated contraction of the heart and, on the other hand, an atrial stimulation that is generated by the pacemaker. For this reason, a heart rate adaptation could, until now, be performed only for a certain trigger eventxe2x80x94for example an atrial and ventricular stimulation of the heart by the pacemakerxe2x80x94based on a single at-rest curve. From a physiological point of view this is insufficient.
Based on the above, the object of the invention is to improve a method for determining a variable control parameter for an implantable medical device in such a way that different initial situations and the resulting influences on the determination of the parameter are compensated for.
This object is met with a method comprising the following combination of procedural steps:
measuring a physiological base parameter that is significant for the control parameter, after a trigger event has occurred,
determining a measuring signal curve for the base parameter from the above measuring process,
determining a certain event type of the trigger event,
selecting a reference signal curve in dependence on the determined event type,
comparing the measuring signal curve to the selected reference signal curve,
determining a comparison value that is representative for the difference between the measuring signal curve and the reference signal curve, and
determining the control parameter from the comparison value according to a predefined calculation algorithm.
In addition to determining an active physiological control parameter, such as the aforementioned heart rate of a pacemaker, passive control parameters may be determined as well, which are of interest, for example, as diagnostic data for the recognition of transplant rejection reactions, ischemic episodes, effects of drugs, or the early recognition of tachycardias by means of appropriate implantable devices. The selection of possible base parameters is accordingly wide. Besides evaluating a mechanical behavior, such as the contraction of the heart, by measuring the intracardial impedance, an evaluation may also be performed based directly on an electro-medical parameter, such as the intracardial electrogram, the mono-phase action potential, or the evoked response, as the measuring signal curve for a significant physiological base parameter.
The gist of the present invention, in connection with all of the above applications, is the measure of determining certain event types during the performance of the present method for the trigger event that triggers the determination of the variable control parameter, and selecting, in dependence on the determined event type, a reference signal curve that corresponds to this event type. This improves the starting basis for determining the control parameter, and optimizes it for the given prevailing situation.
The invention, which, because of its wide range of medical applications, was described in a very abstract fashion above, will be described in more detail below based on an example embodiment, with reference to the appended drawings.